The present invention relates to a grease composition. More particularly, the present invention relates to a grease composition and a grease composition-sealed bearing for use in a ball-and-roller bearing built in automobile electric parts and accessories including an alternator, an electromagnetic clutch for an automobile air conditioner, a middle pulley, and an electric fan motor.
In the automobile industry, front engine and front driving (FF) automobiles have been widely used for reducing a size and a weight. Although a space for person in an automobile has been increased, and a space for an engine room has been inevitably decreased. Accordingly, automobile electric parts and accessories including an alternator, an electromagnetic clutch for an automobile air conditioner, a middle pulley, and an electric fan motor should have been small-sized and light-weighed. For example, a decrease in outputs of the small-sized alternator is compensated with a high-speed rotation of a rotor. In addition, whereby the engine room is tightly sealed to decrease a noise, resulting in high temperature in the engine room. It urges the parts to be withstand the high temperature.
The ball-and-roller bearing is built in the electric parts and accessories, and is mainly lubricated with a grease composition. It has been reported that when the electric parts and accessories are rotated at high speed and high temperature, a rolling surface of the ball-and-roller bearing is abnormally peeled, thereby shorting the lifetime of the bearing.
In order to avoid the abnormal peeling on the ball-and-roller bearing, Japanese Patent Laid-Open Application No. 3-210394 discloses a method of adding a passivation agent to a grease composition. International Publication No. WO94/03565 discloses a method of adding an antimony compound or a molybdenum compound to a grease composition to prolong a lifetime of a bearing.
However, it is known that a typical passivation agent, i.e., sodium nitrite, is reacted with a secondary amine under an acid condition to produce N-nitrosoamine. Since the nitrosoamine adversely affect the environment, it is not preferable. There are, however, no alternative materials.
Sodium nitrite also acts as a rust preventative, and is widely used in a grease composition for a ball-and-roller bearing used in automobile parts into which water is penetrated from a bottom of an automobile body during the automobile runs. Thus, an alternative of sodium nitrite is needed.
Other passivation agents, antimony compounds or molybdenum compounds including no sodium nitrite contain unfavorably heavy metals, which may adversely affect human bodies and the environment.